Booting Xen =========== Xen follows the zImage protocol defined for 32-bit ARM Linux[1] and the Image protocol defined for ARM64 Linux[2]. In both cases the recommendation to boot in HYP/EL2 mode is a strict requirement for Xen. The exceptions to this on 32-bit ARM are as follows: Xen does not require the machine type to be passed in r1. This register is ignored (so may be invalid or the actual machine type). Xen does not support the ATAG list and requires Device Tree. Therefore r2 must point to the physical address of device tree block (dtb) in system RAM. NOTE: although Xen uses the zImage protocol there is no compression actually used. This should be transparent to the bootloader. The zImage protocol should still be used and not the stricter "raw (non-zImage)" protocol described in arm/Booting. The exceptions to this on 64-bit ARM are as follows: Xen binary should be loaded in memory below 2 TiB. Booting Guests -------------- Xen supports the legacy image header[3], zImage protocol for 32-bit ARM Linux[1] and Image protocol defined for ARM64[2]. Until Xen 4.17, in case of legacy image protocol, Xen ignored the load address and entry point specified in the header. This has now changed. Now, it loads the image at the load address provided in the header. And the entry point is used as the kernel start address. A deviation from uboot is that, Xen treats "load address == 0x0" as position independent execution (PIE). Thus, Xen will load such an image at an address it considers appropriate. Also, user cannot specify the entry point of a PIE image since the start address cennot be predetermined. Users who want to use Xen with statically partitioned domains, can provide the fixed non zero load address and start address for the dom0/domU kernel. The load address and start address specified by the user in the header must be within the memory region allocated by Xen. Also, it is to be noted that if user provides the legacy image header on top of zImage or Image header, then Xen uses the attributes of legacy image header to determine the load address, entry point, etc. Firmware/bootloader requirements -------------------------------- Xen relies on some settings the firmware has to configure in EL3 before starting Xen. * Xen must be entered in NS EL2 mode * The bit SCR_EL3.HCE (resp. SCR.HCE for 32-bit ARM) must be set to 1. [1] linux/Documentation/arm/booting.rst Latest version: http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/arm/booting.rst [2] linux/Documentation/arm64/booting.rst Latest version: http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/arm64/booting.rst [3] legacy format header Latest version: https://source.denx.de/u-boot/u-boot/-/blob/master/include/image.h#L315 https://linux.die.net/man/1/mkimage